1. Field of Invention
The invention relates to electronic musical instruments and in particular to an electronic organ having multiplexed keying. In such organs input lines containing various information signals are sampled under the control of a fixed clock and time division multiplexed for subsequent use in the organ and for the ultimate production of audio waveforms. The audio output signals from such organs contain distortion caused by passing square wave signals with harmonic components at a frequency above that tolerated by the sampling rate. The present invention determines the input lines actually containing information signals at a particular point in time and accordingly controls the sampling of only those active input lines. The limitation of the sampling to only active input lines increases the sampling rate which in turn increases the frequency of the harmonic component of the square wave signals that can be passed by the system without distortion.
2. Description of the Prior Art
Electronic organs having multiplexed keying are well known, such as the organ described in U.S. Pat. No. 4,227,432 entitled "Electronic Musical Instrument Having Multiplexed Keying" which issued on Oct. 14, 1980 and is made a part of this specification by reference. In the above multiplexed organ various input lines containing harmonic information, namely upper manual drawbar, percussion, lower manual drawbar and pedal drawbar are sampled and time division multiplexed for subsequent use in keyer circuits. In addition, other input lines containing keying information, namely upper manual sustain, upper manual percussion, lower manual and pedals are sampled and time division multiplexed for subsequent use with the time division multiplexed harmonic information in the keyer circuits. The output of the keyer circuits is demultiplexed and applied to a plurality sine filters. The outputs of the sine filters are applied to standard amplifier circuits and speakers to provide an audio output. The multiplexed organ is controlled by a counter circuit which synchronizes the sampling of each of the input lines containing harmonic information with the sampling of the input lines containing keying information and controls the multiplexing and demultiplexing operations.
In the above described system each input line containing harmonic information and the corresponding input line containing keying information is sampled without regard to whether that line actually contains any information signals. The counter circuit controls the sampling, multiplexing and demultiplexing and receives a 1 MHz clock signal, thus the sampling rate for the system with four channels of harmonic information is 250 KHz. In accord with the Nyquist Theorem the highest frequency tolerated without distortion is one-half the frequency of the sample rate or in the above example 125 KHz. If a frequency with harmonic components above the maximum 125 KHz is received, an undesirable distortion known as aliasing occurs. Aliasing is the introduction of error into the Fourier Analysis of a discrete sampling of continuous data when components with frequencies too great to be analyzed with the sampling internal or rate being used contribute to the amplitudes of lower frequency components.
In an electronic organ using a tone signal in the form of a square wave which is rich in harmonic components aliasing causes the harmonic components above the maximum frequency determined by the sampling rate to foldback below the maximum frequency and to occur in the audio output as distortion or spurious signals. This type of distortion is illustrated in the graph of FIG. 1.
In FIG. 1 the coordinate axis represents the amplitude of the harmonics and the abscissa axis represents the frequency. The graph of FIG. 1 is not drawn to scale and is intended only to illustrate the principle of aliasing. For a signal with a fundamental frequency of a given amplitude A each odd harmonic has a reduced amplitude. For example, the third harmonic has an amplitude approximately one-third of the funamental and the ninth harmonic has an amplitude one-ninth of the fundamental. If the sampling rate is set at 250 KHz for four inputs lines or phases as discussed above, the maximum frequency or Nyquist frequency before aliasing occurs is 125 KHz. If the signal passing through the system has a fundamental frequency of 18 KHz with an amplitude of A, then the third harmonic has a frequency of 54 KHz and an amplitude of 1/3 A. The frequency of the seventh harmonic is 126 KHz and the amplitude is 1/7 A. Since the frequency of the seventh harmonic is greater than 125 KHz, aliasing occurs and the seventh harmonic appears in the output as distortion or a spurious signal occuring at 124 KHz. This distortion caused by aliasing is shown in FIG. 1. The ninth harmonic and the eleventh harmonic also appear in the output as distortion or spurious signals at 88 KHz and 52 KHz respectively, not shown in FIG. 1 for the sake of clarity. When the aliasing frequency components occur within the audible frequency range (approximately 20 Hz through 20 KHz) the audio output signal becomes distorted or contains spurious signals. The listener commonly hears this distortion as a implement sounds noise in the audio output and, of course, this detracts from the listening pleasure. However, the aliasing distortion due to the seventh harmonic appearing as a signal of 124 KHz frequency and a 1/7 A amplitude as well as the distortion caused by the ninth and eleventh harmonic are well outside the audio frequency range. Therefore, the distortion is not considered as a serious impediment to the generation of musical sounds.
This same type of aliasing distortion is also created due to the thirteenth harmonic of the 18 KHz fundamental. The thirteenth harmonic is a signal at 234 KHz with an amplitude of 1/13 A (A is the amplitude of the 18 KHz fundamental). Since the Nyquist frequency is 125 KHz the distortion due to aliasing generates a spurious signal at 16 KHz with an amplitude of 1/13 A. The aliasing distortion for the thirteenth harmonic of the 18 KHz fundamental occurs within the audio frequency range and therefore, the distortion can be heard by the listener and it detracts from the music. Of course, aliasing distortion continues to occur as the harmonics increase, however since the amplitude of the harmonic signal decreases with increasing frequency the distortion caused by higher harmonics is less noticeable.
In prior electronic organs each line containing input information is sampled even though no data is on the line. As a result the sampling rate is fixed and aliasing occurs at the frequency determined by the Nyquist theorem. If input information is only on two of the four input lines and all four lines are nevertheless sampled, the sample rate is half of that actually required and aliasing occurs at a frequency much lower than necessary. Furthermore, since the amplitude of the harmonics decreases with increasing frequency the lower the Nyquist frequency the greater and more noticeble is the distortion.
It is a general object of the present invention to provide a dynamic controller circuit for use in an electronic organ having multiplexed keying for reducing distortion in the audio signal caused by aliasing.
It is a specific object of the present invention to provide a dynamic controller for use in an electronic organ having multiplexed keying and a plurality of lines at least some of which having a harmonic content signal at any point in time for sensing and sampling only those lines containing a harmonic content signal from the plurality of lines to effectively increase the sample rate if all lines do not contain a harmonic content signal.